The watershed of the Mississippi River drains 41 percent of the contiguous United States and includes waters from several major river systems, including the Missouri-Platte River Basin, the Ohio-Tennessee River Basin, and the Arkansas-Red-White River Basin. The Mississippi River basin includes two ffinctionally distinct zones, each with its own potential to contribute to the reduction of Gulf hypoxia. These zones include the huge Mississippi watershed with its tributary network, and at the lower end of the river system, the deltaic zone that formerly dispersed river water naturally throughout SE Louisiana via a distributary (deltaic) network. While the tributaries of the Mississippi River are the sources of nutrient loading to the river trunk, the distributaries within the Mississippi Delta are critical to the final dispersal of nutrients and sediments into the Gulf of Mexico, and the salinity of the estuaries and coastal waters.
The distributary zone includes the entire area influenced by river flow south of the Old River Control Structures, where the Atchafalaya River diverges from the lower Mississippi River and the Red River merges with the Atchafalaya (Figure 1). During the past two centuries the hydrology of the distributary zone was totally modified by the construction of flood levees and closing of key distributaries for flood control and navigation enhancement programs. These structures isolated the river from its delta, causing an ongoing catastrophic collapse in the deltaic landscape, primarily wetlands. The hydrologic changes that have caused such damage to South Louisiana also exacerbate Gulf hypoxia by jetting most nutrient-rich river water and sediments directly into the Gulf of Mexico, bypassing the deltaic wetlands that require the nutrients and sediments.
States and Tribes within the entire Mississippi/Atchafalaya River Basin and Federal agencies are working together to take action to reduce the size of the hypoxic zone, while protecting and restoring the human and natural resources of the Mississippi River Basin. In January 2001, the Mississippi River/Gulf of Mexico Watershed Nutrient Task Force issued the Action Plan for Reducing, Mitigating, and Controlling 1-lypoxia in the Northern Gulf of Mexico. The plan stimulated a great deal of collaboration in understanding science and planning; however, much work remains to be done to implement this plan.